The Caltech researchers who posited Planet Nine's existence based their hypothesis on the unusual movement of six large objects floating in the Kuiper belt. The strange movement patterns suggested that the objects' orbits were being shaped by a hidden planet.

Now, a team of astronomers has performed new calculations with the data that originally tipped them off to Planet Nine's existence - and the new numbers suggest that there could be multiple planets hiding at the edge of our solar system that are just waiting to be discovered.

This new team of astronomers suggests that these Kuiper belt objects (KBOs), which are specifically classified as ETNOS, or objects that orbit the sun beyond Neptune, might not be as stable as we previously thought. "With the orbit indicated by the Caltech astronomers for Planet Nine, our calculations show that the six ETNOs... would move in lengthy, unstable orbits," says Carlos de la Fuente Marcos, an astronomer on the team.

In layman's terms, the gravitational effect that Planet Nine by itself would have on these KBOS would make them too unstable to be moving the way the Caltech scientists think they are. "These objects would escape from the solar system in less than 1.5 billion years," explains de la Fuente Marcos, "What's more important, their orbits would become really unstable in just 10 million years, a really short amount of time in astronomical terms."

According to de la Fuente Marcos and his fellow researchers, it's possible that the relative stability of these objects might instead be due to the gravitational pull of not one, but multiple planets. But not everyone is convinced. Astronomer Mike Brown, a member of the original Caltech team, says "I think it's way too early to start speculating about a second planet, but, in general, I am confused by their results. We have a nearly identical analysis which shows nearly the opposite result. It is not obvious to me why they would get such a different answer."

"The possibility of a new planet is certainly an exiting one for me as a planetary scientist and for all of us. This is not, however, the detection or discovery of a new planet. It's too early to say, with certainty, that there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."

With this much research being done on Planet Nine (and potential planets Ten and Eleven), we're really looking forward to that "exciting result" in the near future.